That's because Wurzer, as president of cannabis-testing company SC Labs, knows how prevalent the use of health-threatening chemicals are in an industry that until recently operated mostly in the shadows. Three to four of every 10 samples that SC tests reveal the presence of pesticides that shouldn't be used on cannabis, including one that turns into a poisonous gas when ignited, he said.

"I don't want some farmer with no one looking over their shoulder spraying away all kinds of pesticides that they don't really understand, that they are not really trained to use," said Wurzer, who has a doctor's prescription to buy pot. "I choose to get cannabis from people I know aren't using pesticides."

With 29 states and the District of Columbia now allowing medical or recreational cannabis use, the $6 billion U.S. industry is expected to reach $50 billion by 2026, according to investment bank Cowen & Co. Yet many states are just beginning to check for pesticides, while some have no testing program at all. California, where voters approved medical marijuana 20 years ago, won't begin testing for impurities until recreational sales begin Jan. 1.

Regulating pesticides for marijuana plants is a state issue because the federal government still classifies pot as an illegal drug. That's prevented the Environmental Protection Agency from evaluating the safe use of cannabis pesticides, as it does for all other crop chemicals. An attempt by Scotts Miracle-Gro Co. to register some pesticides for pot was rebuffed in June by EPA Administrator Scott Pruitt.

Colorado last year found that 49 percent of cannabis samples tested in response to complaints had residue of unapproved pesticides, according to state Department of Agriculture data. The failure rate so far this year is 13 percent, demonstrating how testing improves the safety of marijuana, said Jan Stapleman, a spokeswoman for Colorado's Department of Public Health and Environment.

Oregon also is seeing an improvement after mandatory testing that started late last year resulted in hundreds of failures, said Jonathan Modie, a spokesman for the state Health Authority.

The advent of state-regulated cannabis testing has gone a long way toward ensuring legal pot can "reasonably be considered safe," said Whitney Cranshaw, an entomologist and pesticide expert at Colorado State University. "Prior to that point, we were in a Wild West period with pesticide use, since not only the federal agencies but also the state agencies avoided the issue entirely."

Testing is only now getting started because marijuana was illegal for so long, said Natalie Darves, who runs Cougar Acres Consulting and teaches at Oaksterdam University in Oakland, California, which bills itself as America's first cannabis college. The long prohibition and current quasi-legal status has politicized marijuana, making states hesitant to step into the federal government's regulatory role, she said.

When they do step in, states struggle with implementing a pot-testing program, said Julianne Nassif, director of environmental health at the Association of Public Health Laboratories. Lacking federal guidance, they must develop regulations, license private labs if they aren't going to test samples in state facilities and determine what types of pesticides, pathogens, fungal toxins and heavy metals to monitor. And funding is always an issue, she said.

Cannabis regulation is further complicated by the dizzying number of ways pot can be consumed in recreational markets like Colorado, said John Scott, pesticide section chief for the state agriculture department. In addition to smoking, cannabis can be ingested through baked goods, eye drops, skin lotions and even suppositories, Scott said. Smokable extracts not only concentrate marijuana's active ingredients, but also any pesticide residue, he said.

"Marijuana is unlike any other agricultural crop that we have ever dealt with," Scott said.

Three pesticides accounted for more than one-third of about 900 cannabis failures in Oregon since October, according to state testing data compiled by Bloomberg. The most common was spinosad, a relatively safe insecticide used to control caterpillars on crops such as corn, cotton and tobacco. The second leading cause of failure was piperonyl butoxide, a chemical used with other insecticides to boost effectiveness that the EPA says is a possible human carcinogen.

Myclobutanil, the third-most-common pesticide detected, can be dangerous on cannabis because heating the chemical creates hydrogen cyanide, a poisonous gas. In Canada, where medical marijuana is legal nationwide, the discovery of myclobutanil and piperonyl butoxide residue in cannabis this year prompted product recalls and the start of a mandatory testing program.

Sold by Dow Chemical Co. as Eagle 20, myclobutanil is typically used to combat powdery mildew. But it's unnecessary for cannabis when growers can turn to safe solutions such as Regalia, a nontoxic treatment for the pest, Oaksterdam's Darves said.

Regalia is made by Marrone Bio Innovations Inc., a publicly traded producer of biopesticides. Pam Marrone, chief executive officer of Marrone Bio, said she had no plan to supply pot farmers when Regalia became a hot topic in online chat rooms. Growers flooded customer-service lines with requests for the plant-based product. So Marrone hired a distributor, Vital Garden Supply, to sell Regalia, Grandevo and other biopesticides to the cannabis market.

"They were a little nervous about going into the cannabis industry," said Paul Harton, a Vital sales manager. "We sell a phenomenal amount of their products, and it's growing exponentially."

Some businesses are catering to customers concerned about pesticides by offering so-called clean cannabis. Amy Andrle, co-owner of L'Eagle Services, a Denver dispensary and grower, said non-toxic treatments like neem oil and garlic sprays -- along with dehumidifiers and high-grade air systems -- help her prevent common pot pests like powdery mildew. She also uses beneficial bugs and biological treatments to control damaging insects. Third-party tests provide proof to customers that L'Eagle weed is pesticide-free.

Finding clean-cannabis suppliers may be the best route for pot users concerned about pesticide residues in states where there is no testing regime. Arizona, for instance, has no plans to establish such a program because that wasn't part of the voter-approved referendum allowing medical marijuana, said Holly Ward, a spokeswoman for the state Department of Health Services. More than 137,000 residents are enrolled in Arizona's medical marijuana program.

Wurzer, who runs the California testing lab, said he's relieved consumers won't need to worry about hazards in their weed once the state implements its program in the new year.

"It's really a good thing for the general cannabis consumers that California is regulating now," Wurzer said. "People can assume that someone is actually looking out for them."